1. Field of the Invention
The present invention relates to methods of preventing and eliminating the formation of trisulfide bonds in proteins during protein production and purification procedures.
2. Background Art
Recombinant proteins, and in particular, monoclonal antibodies (mAbs), have become an important class of therapeutic compounds employed for the treatment of a broad range of diseases. Recent successes in the field of biotechnology have improved the capacity to produce large amounts of such proteins. However, extensive characterization of the products demonstrates that the proteins are subject to considerable heterogeneity. For example, molecular heterogeneity can result from chemically-induced modifications such as oxidation, deamidation, and glycation as well post-translational modifications such as proteolytic maturation, protein folding, glycosylation, phosphorylation, and disulfide bond formation. Molecular heterogeneity is undesirable because therapeutic products must be extensively characterized by an array of sophisticated analytical techniques and meet acceptable standards that ensure product quality and consistency.
Antibodies (or immunoglobulins) are particularly subject to such structural heterogeneity due to the fact that they are large, multi-chain molecules. For example, IgG antibodies are composed of four polypeptide chains: two light chain polypeptides (L) and two heavy chain polypeptides (H). The four chains are typically joined in a “Y” configuration by disulfide bonds that form between cysteine residues present in the heavy and light chains. These disulfide linkages govern the overall structure of the native H2L2 tetramer. Overall, IgG1 antibodies contain four interchain disulfide bonds, including two hinge region disulfides that link the H chains, and one disulfide bond between each heavy H and L chain. In addition, twelve intrachain disulfide linkages may involve each remaining cysteine residue present in the molecule. Incomplete disulfide bond formation, or bond breakage via oxidation or beta-elimination followed by disulfide scrambling, are all potential sources of antibody heterogeneity. In addition, a further type of modification, namely trisulfide (—CH2—S—S—S—CH2—) bond formation, was recently reported within the interchain, hinge region bonds of a human IgG2 antibody. See, Pristatsky et al., Anal. Chem. 81: 6148 (2009).
Trisulfide linkages have previously been detected in superoxide dismutase (Okado-Matsumoto et al., Free Radical Bio. Med. 41: 1837 (2006)), a truncated form of interleukin-6 (Breton et al., J. Chromatog. 709: 135 (1995)), and bacterially expressed human growth hormone (hGH) (Canova-Davis et al., Anal. Chem. 68: 4044 (1996)). In the case of hGH, it was speculated that trisulfide formation was promoted by H2S released during the fermentation process. See, International Published Patent Application No. WO 96/02570. Consistent with this hypothesis, the trisulfide content of hGH was increased by exposure to H2S in solution. See, U.S. Pat. No. 7,232,894. In addition, exposing bioreactor material or bacterial lysate to an inert gas inhibited trisulfide formation, apparently by stripping H2S from the system. See, International Published Patent Application No. WO 2006/069940.
Conditions that influence the level of trisulfide in hGH generated during purification from bacteria have previously been reported. For example, the presence of alkali metal salts has been reported to inhibit increases in trisulfide bonds during downstream protein processing. See, U.S. Pat. No. 7,232,894. Furthermore, treatment of hGH with reduction-oxidation (REDOX) compounds, including L-cysteine, in solution was found to convert trisulfide bonds to disulfide bonds. See, International Published Patent Application No. WO 94/24157. For example, treatment of purified IgG2 with a 20-fold molar ratio of L-cysteine in solution was found to convert hinge region trisulfide bonds to disulfides during sample preparation for analytical studies. See, Pristatsky et al., Anal. Chem. 81: 6148 (2009).
Unfortunately, removal of trisulfide bonds by exposure to cysteine in solution has several drawbacks, in particular for large scale processing. For example, large quantities of cysteine are required. This method also necessitates a separate step to remove cysteine from the sample after trisulfide bonds are removed. In addition, removal of trisulfide bonds by exposure to cysteine in solution can promote aggregation through the formation of undesirable disulfide linkages. Therefore, in order to address the limitations of previous methods for reducing trisulfide bonds, the methods described herein provide efficient and improved means for preventing and eliminating the formation of trisulfide bonds in proteins (such as, for example, in antibodies) during production and purification procedures used in the manufacture of such proteins.